The fabrication of integrated circuits includes numerous processes. Ion implantation is one such process commonly used in the manufacture of integrated circuits wherein dopant ions are selectively implanted, for example, through an organic photoresist mask into the surface of a semiconductor substrate or wafer. The photoresist mask is typically cast from a solvent and developed into a patterned mask using a photolithography process and may be used as a patterned mask during an ion implantation process. During an ion implantation process, the dopant ions or implant species react with the surface of the photoresist mask, and are implanted into the semiconductor substrate through the openings in the photoresist mask.
The implant species also becomes embedded in the patterned photoresist material during the ion implantation. Implant species (containing dopant ions) may include, but are not limited to, elements such as arsenic, phosphorus, and boron. When the implant process has been completed, the photoresist mask is typically removed or stripped using either a conventional wet or dry stripping process. One form of dry stripping is commonly referred to as ashing, and in a typical ashing process, a wafer is placed into an oxygen plasma asher to break through the implant species (dopant) crusted portion of the photoresist layer and remove the patterned photoresist mask. Plasma asher devices include downstream plasma ashers, microwave plasma ashers, or inductively coupled plasma reactors or chambers.
An oxygen plasma asher may contain a variety of reactive gases, or forming gases, such as oxygen, hydrogen, nitrogen, or fluorine. The reactive gases in the plasma chamber remove the photoresist material and implant species by forming volatile reaction products and/or by weakening the adhesion of the photoresist to the substrate or wafer. The dopant ions or implant species will react with the reactive gases, for example, hydrazine (N2H2), that are fed into the asher.
Typically, a ratio of oxygen to N2H2 is 6.6 to 1 using an oxygen flow rate of 2,000 sccm (standard cubic centimeters per minute) and an N2H2 flow rate of 300 sccm. In a typical resist ash recipe, it is desirable to increase the resist ash rate. It has been shown, for example in Fujimura et al. “Additive Nitrogen Effects On Oxygen Plasma Downstream Ashing” (Japanese Journal of Applied Physics, Vol. 29, No. 10, October 1990, pp 2165-2170), that additional nitrogen in oxygen plasma could increase a resist ash rate. In U.S. Pat. No. 6,524,936 by Hallock et al., in an ashing process, the photoresist mask is exposed to ultra-violet light, allowing increased temperatures used during an ashing process, resulting in faster throughputs.
The stripping process normally continues until the photoresist has been removed or the photoresist residues are rendered removable by a wash or rinse step. While it is desirable to increase the asher rate, the formation of compounds with reactive gases must also be increased. It is also desirable to minimize implant species (dopant) deposits in the plasma asher chamber. Reactive gases and implant species form volatile compounds that are pumped away or out of the plasma asher chamber by the asher chamber vacuum system. Numerous articles, for example, “Chamber Contamination in Ashing Processes of Ion Implanted Photoresist” by Laurent Kassel and Jeff Perry (S.P.I.E., Vol. 1803 (1992)/89, 0-8194-1001-2/93) discuss a variety of methods to reduce or prevent the accumulation of an implant species in a resist asher chamber.